Lrig1 Marks a Distinct Population of Proliferative and Quiescent Intestinal Stem Cells and Acts as a Tumor Suppressor

Whether proliferative and quiescent intestinal stem cells coexist in normal and cancerous tissue is controversial. Due to heightened transcript expression in the intestinal epithelial stem cell compartment and detection in quiescent hair follicle stem cells, we hypothesized that Leucine-rich repeats and immunoglobulin-like domain protein 1 (Lrig1) would mark a novel population of intestinal stem cells and may be important for intestinal homeostasis and in cancer. To this end, we generated Lrig1-CreERT2/+; R26R-LacZ mice for Lrig1 intestinal epithelial cell lineage analysis. Examining adult mice at baseline and after irradiation damage, we show that Lrig1 labels both quiescent and proliferative stem cells and these cells are distinct from the well-defined Lgr5-expressing proliferative stem cell population. In addition, in a regenerative response to gut injury, these singly-labeled Lrig1-expressing cells proliferated and gave rise to clusters of labeled daughter cells, demonstrating the ability of these quiescent stem cells to become activated. Interestingly, loss of Lrig1 resulted in heightened expression of ErbB1-3 in the normal intestine, duodenal adenomas and carcinoma, supporting a role for Lrig1 in the maintenance of intestinal epithelial homeostasis and its ability to act as a tumor suppressor. To directly compare the role of the Lrig1and Lgr5expressing cell populations in tumorigenesis, we used both Lrig1-CreERT2/+;Apcfl/fl and Lgr5-EGFP-IRES-CreERT2;Apcfl/fl mice to delete Apc. Five days after deletion, Lrig1CreERT2/+;Apcfl/fl mice have increased duodenal size and marked histological changes, which are absent in the Lgr5-EGFP-IRES-CreERT2;Apcfl/fl mice, further supporting a distinct and important role for the Lrig1 population in maintaining intestinal homeostasis. Finally, stochastic loss of Apc in Lrig1-expressing cells in Lrig1-CreERT2/+;Apcfl/+ mice results in multiple, large distal colonic tumors and less frequent, smaller intestinal tumorscommon features of human familial polyposis4.5 months after Cre activation. In summary, we show that Lrig1 is a distinct marker of both proliferative and quiescent intestinal stem cells, is important for intestinal homeostasis and acts as a tumor suppressor.